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Herbaceous Layer Species Richness of Southeastern Forests and Woodlands: Patterns and Causes
Abstract
This chapter examines species richness patterns in the herbaceous layer vegetation of forests and woodlands of southeastern North America. It demonstrates the changing importance of various drivers of species richness across environmental and geographic gradients and across vegetation types, and shows how the relative importance of those drivers varies with scale of observation. The most important processes structuring species diversity patterns in southeastern forests and woodlands appear to be cation availability (environmental favorableness), disturbance (flooding, fire, grazing), mass effects, and the relative size of the species pool.
... Herbaceous taxa represent more than 80 percent of plant species in temperate forests with high densities and diversities of these herbs co-occurring within small areas (Gilliam 2007;Peet et al. 2014). Spatial resource partitioning and environmental heterogeneity are the most widely cited mechanisms facilitating coexistence among forest herbs (Bell et al. 2000;Bartels and Chen 2010;Beatty 2014;Catella et al. 2019;Beck and Givnish 2021). ...
... Spatial resource partitioning and environmental heterogeneity are the most widely cited mechanisms facilitating coexistence among forest herbs (Bell et al. 2000;Bartels and Chen 2010;Beatty 2014;Catella et al. 2019;Beck and Givnish 2021). Forest herb distributions vary in response to environmental variation at a variety of spatial scales (Curtis 1959;Struik and Curtis 1962;Beatty 2014;Peet et al. 2014). At local scales, differential plant responses to fine-scale variation in soil depth (Bratton 1976;Beck and Givnish 2021), soil fertility and soil moisture (Collins et al. 1984;Crozier and Boerner 1984;Vellend et al. 2000), microtopography (Beatty 1984;Peterson et al. 1990), light availability (Anderson et al. 1969;Thompson 1980), and other abiotic factors can promote coexistence via spatial resource partitioning. ...
... At local scales, differential plant responses to fine-scale variation in soil depth (Bratton 1976;Beck and Givnish 2021), soil fertility and soil moisture (Collins et al. 1984;Crozier and Boerner 1984;Vellend et al. 2000), microtopography (Beatty 1984;Peterson et al. 1990), light availability (Anderson et al. 1969;Thompson 1980), and other abiotic factors can promote coexistence via spatial resource partitioning. Yet, spatial resource partitioning cannot account for coexistence among many functionally similar species at small spatial scales (Gilliam 2007;Beatty 2014;Peet et al. 2014;Beck 2020). Potential interactions among herbaceous plants and soil biota could shape many understory community dynamics and potentially promote local coexistence, but these require further study in temperate forest understories (Whigham 2004;Comita et al. 2014). ...
Antagonistic interactions between plants and soil biota promote species diversity in many plant communities but little is known about how these plant–soil interactions influence herbaceous species in temperate forests. To assess the potential for soil biota to affect the growth of forest herbs, I conducted a greenhouse experiment in which seedlings of nine focal herb species common in Wisconsin (USA) forests were grown in soil derived from conspecific and heterospecific plants. This soil origin treatment was crossed with a subsequent treatment in which half of the soils were pasteurized to eliminate soil biota. The presence and origin of soil biota had variable effects on plant growth among the nine focal species. Thalictrum dioicum, Elymus hystrix, and Solidago flexicaulis growth were inhibited by the presence of soil biota in unpasteurized soils. Thalictrum dioicum seedlings grown in conspecific, unpasteurized soil accumulated 30% less biomass than seedlings grown in heterospecific, unpasteurized soil indicating that host-specific effects of microbial pathogens restrict seedling growth. Similarly, E. hystrix seedlings were 11% smaller in conspecific-trained soils. The remaining herb species showed no significant response to experimental treatments manipulating soil biota. These variable growth responses highlight the potential for differences in plant–soil interactions among plant species to influence local plant distributions and community dynamics. Janzen–Connell effects, like those observed in T. dioicum and E. hystrix, could promote coexistence among certain species and contribute to high local plant diversity in temperate forest understories.
... Forest herbs share general characteristics that are well adapted to forest understory conditions but also differ greatly in key morphological and physiological characteristics reflecting the diverse habitats they occupy (Whigham, 2004). For example, herb distributions respond sensitively to edaphic conditions and light availability (Curtis, 1959;Givnish, 1982Givnish, , 1987Leach and Givnish, 1999;Gilbert and Lechowicz, 2004;Peet et al., 2014). Species turnover along these environmental gradients reflects physiological tradeoffs, suggesting that ecological sorting structures herb distributions both at local scales (Beatty, 2014;Beck and Givnish, 2021) and across landscapes (Curtis, 1959;Amatangelo et al., 2014;Peet et al., 2014;Rolhauser et al., 2021). ...
... For example, herb distributions respond sensitively to edaphic conditions and light availability (Curtis, 1959;Givnish, 1982Givnish, , 1987Leach and Givnish, 1999;Gilbert and Lechowicz, 2004;Peet et al., 2014). Species turnover along these environmental gradients reflects physiological tradeoffs, suggesting that ecological sorting structures herb distributions both at local scales (Beatty, 2014;Beck and Givnish, 2021) and across landscapes (Curtis, 1959;Amatangelo et al., 2014;Peet et al., 2014;Rolhauser et al., 2021). The potential for trait differences and limiting similarity to stabilize local coexistence among forest herbs has received less attention. ...
Premise: Numerous processes influence plant distributions and co‐occurrence patterns, including ecological sorting, limiting similarity, and stochastic effects. To discriminate among these processes and determine the spatial scales at which they operate, we investigated how functional traits and phylogenetic relatedness influence the distribution of temperate forest herbs.
Methods: We surveyed understory plant communities across 257 forest stands in Wisconsin and Michigan (USA) and applied Bayesian phylogenetic linear mixed‐ effects models (PGLMMs) to quantify how functional traits and phylogenetic relatedness influence the environmental distribution of 139 herbaceous plant species along broad edaphic, climatic, and light gradients. These models also allowed us to test how functional and phylogenetic similarity affect species co‐occurrence within microsites.
Results: Leaf height, specific leaf area, and seed mass all influenced individualistic plant distributions along landscape‐scale gradients in soil texture, soil fertility, light availability, and climate. In contrast, phylogenetic relationships did not consistently predict species‐environment relationships. Neither functionally similar nor phyloge- netically related herbs segregated among microsites within forest stands.
Conclusions: Trait‐mediated ecological sorting appears to drive temperate‐forest community assembly, generating individualistic plant distributions along regional environmental gradients. This finding links classic studies in plant ecology and prior research in plant physiological ecology to current trait‐based approaches in community ecology. However, our results fail to support the common assumption that limiting similarity governs local plant co‐occurrences. Strong ecological sorting among forest stands coupled with stochastic fine‐scale interactions among species appear to weaken deterministic, niche‐based assembly processes at local scales.
... Forest herbs share general characteristics that are well adapted to forest understory conditions but also differ greatly in key morphological and physiological characteristics reflecting the diverse habitats they occupy (Whigham, 2004). For example, herb distributions respond sensitively to edaphic conditions and light availability (Curtis, 1959;Givnish, 1982Givnish, , 1987Leach and Givnish, 1999;Gilbert and Lechowicz, 2004;Peet et al., 2014). Species turnover along these environmental gradients reflects physiological tradeoffs, suggesting that ecological sorting structures herb distributions both at local scales (Beatty, 2014;Beck and Givnish, 2021) and across landscapes (Curtis, 1959;Amatangelo et al., 2014;Peet et al., 2014;Rolhauser et al., 2021). ...
... For example, herb distributions respond sensitively to edaphic conditions and light availability (Curtis, 1959;Givnish, 1982Givnish, , 1987Leach and Givnish, 1999;Gilbert and Lechowicz, 2004;Peet et al., 2014). Species turnover along these environmental gradients reflects physiological tradeoffs, suggesting that ecological sorting structures herb distributions both at local scales (Beatty, 2014;Beck and Givnish, 2021) and across landscapes (Curtis, 1959;Amatangelo et al., 2014;Peet et al., 2014;Rolhauser et al., 2021). The potential for trait differences and limiting similarity to stabilize local coexistence among forest herbs has received less attention. ...
Premise
Numerous processes influence plant distributions and co‐occurrence patterns, including ecological sorting, limiting similarity, and stochastic effects. To discriminate among these processes and determine the spatial scales at which they operate, we investigated how functional traits and phylogenetic relatedness influence the distribution of temperate forest herbs.
Methods
We surveyed understory plant communities across 257 forest stands in Wisconsin and Michigan (USA) and applied Bayesian phylogenetic linear mixed‐effects models (PGLMMs) to quantify how functional traits and phylogenetic relatedness influence the environmental distribution of 139 herbaceous plant species along broad edaphic, climatic, and light gradients. These models also allowed us to test how functional and phylogenetic similarity affect species co‐occurrence within microsites.
Results
Leaf height, specific leaf area, and seed mass all influenced individualistic plant distributions along landscape‐scale gradients in soil texture, soil fertility, light availability, and climate. In contrast, phylogenetic relationships did not consistently predict species‐environment relationships. Neither functionally similar nor phylogenetically related herbs segregated among microsites within forest stands.
Conclusions
Trait‐mediated ecological sorting appears to drive temperate‐forest community assembly, generating individualistic plant distributions along regional environmental gradients. This finding links classic studies in plant ecology and prior research in plant physiological ecology to current trait‐based approaches in community ecology. However, our results fail to support the common assumption that limiting similarity governs local plant co‐occurrences. Strong ecological sorting among forest stands coupled with stochastic fine‐scale interactions among species appear to weaken deterministic, niche‐based assembly processes at local scales.
... Therefore, it is not surprising that richness would increase over the year following the fire event, especially as pulses of seeds from the species pool could be increasing dispersal following fire disturbances (Harms et al., 2017;L. K. Kirkman et al., 2016;Peet et al., 2014). Alternatively, in sites with longer time since fire, such as 4 or 10 years, we would expect richness to decrease as woody shrubs begin to dominate, and this trend has been documented in other studies (Beckage et al., 2009;Palmquist et al., 2014;Peet et al., 2018). ...
... Although factors such as elevation gradients and soil composition, texture and moisture are important edaphic factors influencing plant cover, richness and overall community composition (Carr et al., 2009;Drewa et al., 2002b;L. K. Kirkman et al., 2001;Mitchell et al., 1999;Peet, 2006;Peet et al., 2014), unfortunately, soil texture and moisture were not measured in this study. Within our xeric study sites, the unexplained variation in above-ground biomass, wiregrass cover and species richness (Figure 3) is probably due to differences in soil moisture. ...
Questions
Restoration of ecosystems is complex, with multiple targets that can work in concert or conflict with each other, such as biodiversity, species dominance and biomass. When properly managed, longleaf pine (LLP) savannas are among the most biologically diverse habitats in the world. However, anthropogenic influences, such as fire suppression, have decimated this ecosystem and its biodiversity, making restoration a priority. Here, we describe the biodiversity and community dynamics seen in the understory layer across xeric LLP savannas in North Carolina and then answer the following questions: What are the predictors of (1) biodiversity, (2) dominance and (3) biomass at multiple spatial scales?
Location
Fifteen observational study sites in North Carolina spanning from the Sandhills to the Coastal Plain.
Methods
At each of the 15 sites, 25 sampling plots were established where above‐ground herbaceous biomass, species presence and abundance, soil characteristics and light availability were measured along with numerous other environmental variables.
Results
Considerable variation exists across study plots within and across sites, with plant species richness ranging from 1 to 17 per m ² . The relative cover of the dominant grass species, Aristida stricta (wiregrass), also varied greatly within and across sites, with a median of ca. 30% relative cover per plot. Wiregrass was a significant predictor of biomass and biodiversity at small scales. With increasing wiregrass abundance, richness decreases, with 25% relative wiregrass cover leading to the highest levels of biodiversity. Likewise, because wiregrass abundance is one of the stronger predictors of above‐ground biomass, we also found a unimodal richness–biomass relationship.
Conclusions
Our results indicate that at lower ends of the productivity and richness gradients, land managers can increase all three restoration targets in the understory at the same time; however, at more diverse and productive sites, restoration practitioners may need to prioritize one target or find a balance between all three.
... Variation in plant traits or functional types can be related to variation in soil nutrients, texture, and moisture (e.g., Rocarpin et al. 2016). In southeastern coastal plain pine savannas with high fire recurrence, three edaphic properties (moisture, texture, and base cation exchange) are important determinants of species richness at various spatial scales (Peet et al. 2014, Palmquist et al. 2015. Palmquist et al. (2015) determined that silty, FIG. 4. Nonmetric multidimensional scaling plots of plant community composition (presence-absence) with significant axes from principal components analysis of subsurface soil in subtropical calcareous wet grasslands and seven other community types of Avon Park Air Force Range, FL as defined by Orzell and Bridges (2006c). ...
... Herb layers are usually sparse, with only a few shade-tolerant species frequent, and then usually with low cover. In addition to reduced insolation, the low level of basic cations and typically dry upland soils limit the herb layer cover and diversity (Peet et al. 2014;Hakkenberg et al. 2020); in some places a high population density of white-tailed deer further limits development of both shrub and herb cover due to heavy browsing. ...
The Piedmont (PDMT) ecoregion of the USA stretches from New Jersey to Alabama, nestled between the Coastal Plain and Blue Ridge Mountain physiographic provinces. Many of the notable Piedmont plant communities, including the dominant oak-hickory forests of the region, are reliant upon fire to some degree. Before human settlement, most Piedmont vegetation burned relatively frequently and at low intensities, resulting in extensive closed canopy oak-hickory forests, studded with patches of open woodland and savanna largely defined by unusual soil conditions. Indigenous peoples of the Piedmont used fire as a land management tool for both agriculture and game production. Historical changes in land use throughout the region have altered fire regimes and changed forest dynamics dramatically over the past 400 years. Euro-American settlement led to widespread clearing of land for agriculture and logging; by the early twentieth century, very little old-growth forest remained in the Piedmont. During the mid-twentieth century, the decline of agriculture and the aggressive suppression and exclusion of wildfires brought about the growth of successional forests in the place of older, fire-mediated communities. The Piedmont region is currently experiencing a rapid expansion of the human population and land development, making restoration of the historical fire regime a challenge. However, land managers frequently do use prescribed fire to enhance timberland and restore rare plant communities.
... Riparian shrubland with Alnus serrulate and Xanthorhiza simplicissima growing along Blue Ridge Mts. rivers are probably the most species-rich woody communities in temperate North America, harboring as many as 129 plant species per 100 m 2 (Peet et al., 2014). Thus, understanding the drivers and spatial patterns of this extraordinary species richness is of vital importance. ...
This study aimed to analyze plant species richness in riparian forests at both local and regional scales across several watersheds in the Sudetes (Poland, Central Europe). Specifically, species richness in riparian forest was compared to other forest types in the same region. It was also hypothesized that due to high complexity and dynamics, riparian forests share a higher number of rare species. In addition, the longitudinal pattern of species richness was analyzed at both local and regional scales. Finally, the effect of topography on species richness in riparian forests in spring areas and along rivers of various sizes was analyzed.
Riparian forests have significantly higher alpha diversity than beech and ravine forests, but oak forests showed a similar level of diversity. However, a comparison of accumulation curves showed, that riparian forests are the most species-rich at a regional scale. All forest types had a similar share of rare species. Eight uniform groups of rare species were distinguished in riparian forests and reflected the riparian complexity and dynamics.
The number of plant species per plot was highest in spring areas and decreased from headwaters to lower reaches. The estimated total number of species showed a similar pattern; however, the highest number was estimated for riverine forests along 3rd order streams and therefore suggest a unimodal pattern of gamma diversity along a longitudinal (upstream–downstream) gradient. The effect of topographic variables on species richness differed depending on the position in the river network.
... Pine savanna ecosystems of the southeastern USA are extremely biodiverse (Engstrom 1993, Lubertazzi and Tschinkel 2003, Means 2007, Noss 2013, Peet et al. 2014), yet extremely threatened (Frost 2007). In much of the remaining southeastern pine savanna, the canopy and understory vegetation have been greatly altered since European settlement, largely from fire suppression, plowing, and hardwood encroachment (Van Lear et al. 2005). ...
Where historical fire regimes have been disrupted, reduction in woody vegetation is often used to maintain or restore habitat for grassland and early successional birds. In pine savanna ecosystems of the southeastern USA, mechanical hardwood canopy reduction can restore pine savanna communities and is often employed on privately owned lands to improve habitat for the Northern Bobwhite (Colinus virginianus), although scant empirical evidence exists of its effects on target or non‐target species. We measured the response of a pine savanna specialist, the Bachman's Sparrow (Peucaea aestivalis), to large‐scale hardwood reduction in a before–after–control–impact design on two properties where two‐year fire‐return intervals were established and the Bachman's Sparrow population was stable. We investigated the effects of mechanical hardwood reduction on Bachman's Sparrow daily nest survival, cause‐specific nest mortality and adult male annual survival. During the four‐year study, we monitored 107 Bachman's Sparrow nests, recorded 49 nest predation events, and banded 113 adult male Bachman's Sparrows. We found Bachman's Sparrow nest and adult survival were resilient to changes in the hardwood canopy and did not differ significantly between treatment and control sites. Average annual adult male survival was 0.41 (0.32–0.52) and daily survival rate of nests with surveillance declined annually from 0.94 (0.92–0.96) to 0.88 (0.83–0.92). The identity of predators at nests was dominated by two snake species, black racer (Coluber constrictor) and corn snake (Pantherophis guttata). We found evidence for opposing treatment effects on the frequency of nest depredations by the dominant species; racers responded positively and corn snakes responded negatively. Our results suggest a moderate midstory canopy does not limit Bachman's Sparrow vital rates when management includes frequent prescribed fire. Our results also suggest hardwood reduction to mitigate nest predation may be complicated with a diverse predator suite.
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